kite/backend/storage/minio_backend.py

"""
MinIOBackend — synchronous Minio SDK wrapped in asyncio.to_thread().

Every call to the synchronous Minio SDK is offloaded to a thread pool via
asyncio.to_thread() so the FastAPI event loop is never blocked.

Object key schema (STORE-02 / D-06):
    {user_id}/{document_id}/{uuid4()}{ext}

The human-readable filename is NEVER passed into this module — only the
file extension (derived by the caller from Path(original_name).suffix.lower())
reaches here.
"""
from __future__ import annotations

import asyncio
import io
import uuid
from datetime import timedelta
from typing import Optional

from minio import Minio

from storage.base import StorageBackend


class MinIOBackend(StorageBackend):
    """MinIO implementation of StorageBackend.

    All synchronous Minio SDK calls are wrapped in asyncio.to_thread() to
    avoid blocking the FastAPI event loop (RESEARCH.md Pattern 3).
    """

    def __init__(
        self,
        endpoint: str,
        access_key: str,
        secret_key: str,
        bucket: str,
        secure: bool = False,
        public_endpoint: Optional[str] = None,
    ) -> None:
        self._bucket = bucket
        self._client = Minio(
            endpoint=endpoint,
            access_key=access_key,
            secret_key=secret_key,
            secure=secure,
        )
        # MINIO_SERVER_URL on MinIO rewrites the presigned URL host at the server side,
        # so both clients can point at the internal endpoint — the signature is valid
        # for localhost:9000 because MinIO itself generated it that way (T-03-10).
        self._public_client = Minio(
            endpoint=(public_endpoint or endpoint),
            access_key=access_key,
            secret_key=secret_key,
            secure=secure,
        )

    async def put_object(
        self,
        user_id: str,
        document_id: str,
        file_bytes: bytes,
        extension: str,
        content_type: str,
    ) -> str:
        """Store bytes in MinIO and return the generated object key.

        Key schema: {user_id}/{document_id}/{uuid4()}{extension}
        The filename is NOT a parameter — STORE-02 compliance.
        """
        object_key = f"{user_id}/{document_id}/{uuid.uuid4()}{extension}"
        data = io.BytesIO(file_bytes)
        data.seek(0)  # belt-and-braces: BytesIO constructor leaves pointer at 0 already
        await asyncio.to_thread(
            self._client.put_object,
            self._bucket,
            object_key,
            data,
            length=len(file_bytes),
            content_type=content_type,
        )
        return object_key

    async def get_object(self, object_key: str) -> bytes:
        """Fetch object bytes from MinIO by key."""

        def _fetch() -> bytes:
            response = self._client.get_object(self._bucket, object_key)
            try:
                return response.read()
            finally:
                response.close()
                response.release_conn()

        return await asyncio.to_thread(_fetch)

    async def delete_object(self, object_key: str) -> None:
        """Delete an object from MinIO by key."""
        await asyncio.to_thread(self._client.remove_object, self._bucket, object_key)

    async def presigned_get_url(
        self, object_key: str, expires_minutes: int = 60
    ) -> str:
        """Return a time-limited pre-signed download URL."""
        return await asyncio.to_thread(
            self._client.presigned_get_object,
            self._bucket,
            object_key,
            timedelta(minutes=expires_minutes),
        )

    async def health_check(self) -> bool:
        """Return True when the configured bucket is reachable; False on any exception."""
        try:
            return await asyncio.to_thread(self._client.bucket_exists, self._bucket)
        except Exception:
            return False

    async def generate_presigned_put_url(
        self, object_key: str, expires_minutes: int = 15
    ) -> str:
        """Return a presigned PUT URL with a browser-resolvable hostname.

        Generates the URL via the internal client (minio:9000 — reachable from
        the backend container), then rewrites the host to the public endpoint
        (localhost:9000 — reachable from the browser). T-03-10 / Finding 3.
        """
        # MINIO_SERVER_URL on the MinIO container causes it to embed the public
        # hostname (localhost:9000) in signed URLs, so the internal client suffices.
        return await asyncio.to_thread(
            self._client.presigned_put_object,
            self._bucket,
            object_key,
            timedelta(minutes=expires_minutes),
        )

    async def stat_object(self, object_key: str) -> int:
        """Return the authoritative file size in bytes from MinIO stat.

        Calls self._client.stat_object (internal endpoint) and returns .size.
        RESEARCH.md Finding 5: stat_object returns .size as int (authoritative).
        Raises minio.error.S3Error(code='NoSuchKey') if the object does not exist.
        """
        result = await asyncio.to_thread(
            self._client.stat_object, self._bucket, object_key
        )
        return result.size